It is well known that enormous potential stores of oil as well as gaseous hydrocarbons and other gases are contained in certain sedimentary rocks, commonly referred to as oil shale. Upon heating, such rocks yield appreciable quantities of relatively crude oil which may be refined to valuable products such as gasoline, diesel oil, jet fuel and fuel oil. Valuable byproducts such as tar acid and waxes are also recoverable from the crude shale oil. Very extensive deposits of oil shale are located in the United States, particularly in the states of Colorado, Utah and Wyoming, and important oil shale deposits are to be found in various parts of the world. Although even the best oil shales contain only about 0.6 to 0.8 barrels of oil per ton of shale, world conditions coupled with diminishing reserves of oil have led to considerable interest in developing a commercially feasible procedure for processing oil shale to recover its potential yield of crude oil. However, to date, such efforts have been unsuccessful with the result that this immense source of oil remains virtually untapped.
Present technology, upon which most recent research and development effort has been expended, includes mining, crushing and screening the oil shale to provide a particulate feed that can be heated to a sufficiently elevated temperature that a solid organic material within the shale, known as kerogen, can be decomposed by pyrolysis to shale oil, gas and a carbonaceous residue. Shale oil technology based on retorting has not achieved commercial or environmental acceptance for a number of reasons. For one thing, very high retorting temperatures of the order of 800.degree. to 1200.degree. F. or higher are required to carry out the pyrolysis reaction. Not only are the energy requirements enormous, but the decomposition of the organic compounds at such very high temperatures require immense volumes of air. Perhaps more important, the high temperatures contribute directly to pollution and like environmental problems through conversion of the organic materials to sulfides, amines and nitrogen compounds as byproducts from the retort. Effective removal of these pollutants to meet existing pollution standards normally requires use of an afterburner or other device which must be fired by an independent energy source (e.g., natural gas). In addition, the retorting process generally reduces the yield of oil from the shale, both from an inability of the retorting procedure to effectively process "fines" (which must therefore be screened from the shale) and the conversion of certain desirable components to undesirable components at the high temperatures employed. A further and particularly difficult problem is the generally high requirement for process water to effectively carry out the retort procedure. Thus as much as 2.5 barrels of water are required to process 1 barrel of oil from the shale, for purposes of quenching the high temperature residues and condensing volatiles on discharge.
But the foregoing relate only to problems associated with operation of the retort. When the shale has been processed, the residue must be returned to the ground. At the high retort temperatures employed to decompose the shale, irreversible changes take place which provide a further basis for polluting and degrading the land (e.g., contaminating surface waters and runoff passing through the spent shale). The problem of what to do with the poisonous residues from the shale process are, therefore, at least as great as those related to the processing of the byproduct gases and like pollutants.
Various alternative procedures have been proposed, for example, retorting the shale in place in the ground (in situ retorting) but, while minimizing the problems of spent-shale handling, such procedures do not avoid the problems associated with the use of high temperatures, as noted above. Steam distillation has also been proposed (see Egloff U.S. Pat. No. 1,627,162) and also the pyrolitic recovery of shale oil by means of pulsed laser beams within an enclosure from which gaseous products are withdrawn by vacuum pump (see Yant U.S. Pat. No. 3,652,447). However, such procedures have not proved to be successful and have never been commercialized. It is therefore apparent that a relatively simple, low temperature, safe procedure for processing oil shale to recover its oil content is highly to be desired.